Certain medical procedures utilizing flashlamp-excited dye lasers can achieve more optimal results by employing a laser which generates a pulsed output beam having a relatively long pulse duration. One such medical technique is selective photothermolysis, in which targeted tissue which includes a pigmentation abnormality (e.g., a vascular lesion, pigmented lesion or tattoo) is heated by pulses of laser radiation having a wavelength specifically selected to be absorbed by the targeted tissue. The benefits afforded by using a flashlamp-excited dye laser which generates a pulsed output beam having a relatively long pulse duration include improved clearance of the lesion, fewer treatment sessions and reduced incidence of purpura, hypopigmentation and hyperpigmentation.
Conventional flashlamp-excited dye lasers produce pulsed output beams having a pulse duration on the order of 0.1 to 100 microseconds. Attempts to extend the pulse duration typically results in an accumulation of dye molecules in metastable electronic states and thermal distortions in the liquid dye medium (i.e., the gain medium), which cause the laser action (i.e. lasing) to self-terminate. These phenomena are induced, in general, by the excitation energy provided to the dye medium by the flashlamp.
Dye molecules that become excited from a ground state to metastable electronic states during excitation of the dye medium do not return readily to the ground state. Instead, these dye molecules may absorb a photon whose wavelength lies in the operating region the laser pulse. Thus, these molecules no longer contribute to the gain medium and act to inhibit laser action. The addition of cyclotetraene (COT) or diazobicyclooctane (DABCO) to the liquid dye medium effectively quenches these molecules, enabling the laser to generate pulses in excess of a few tens of microseconds. To slow the adverse effects of thermal distortions in the liquid dye medium, a laser with an optical system having a reduced thermal sensitivity can generate pulses up to a few hundred microseconds in duration.
Another limitation to extended pulse durations is the photodegradation of the dye caused by the excitation pulses. There are two aspects to photodegradation: photobleaching which decreases available gain and produces harmful photoproducts. Photobleaching of the dye is when some portion of the dye molecules are destroyed during pulse generation and can no longer participate in the lasing process. The destroyed dye molecules are commonly converted to byproducts that absorb radiation at the laser wavelength and inhibit lasing. As a result of photobleaching, the available gain in the liquid dye medium decreases and the lasing threshold (i.e., the energy level at which oscillation or laser action occurs) rises. As such, the minimum input pulse amplitude (or energy level) needed to achieve the lasing threshold and sustain the pulse rises rapidly. For an input pulse with a constant amplitude, the lasing threshold rapidly exceeds the input pulse amplitude causing laser action to terminate in a few hundred microseconds. A laser utilizing a ramped excitation pulse (i.e., increasing in amplitude during the entire duration of the pulse) to drive the flashlamp can sustain longer pulses in many instances. However, a ramped excitation pulse may not be optimal for a laser utilizing certain dyes.